If I want to work in a cruise line for the first time do I need to have STCW certificates?

It depends on your employer, your role onboard, the vessel's flag and other factors. The general rule is that even if it is not legally required most employers will ask for STCW certificates. So it is best to first check with your employer.

 
SQLearn provides STCW Basic training courses that are requested by most employers, more info about the available courses can be found here.